1. Field of the Invention
The present invention relates to intraluminal endoprosthetic devices known as stents. In particular, the present invention relates to stents having helical elements with a geometry that allows the stent to be readily crimped onto a delivery device.
2. Description of Related Art
Stents are prosthetic devices that are implanted in the lumen of a vessel inside the body to provide support for the vessel""s wall. Structural support from stents is particularly important in angioplasty procedures. Typically, stents are implanted within a vessel system to reinforce vessels that are partially occluded, collapsing, weakened, or abnormally dilated. More generally, stents can be used inside any physiological conduit or duct, includingxe2x80x94for examplexe2x80x94arteries, veins, bile ducts, the urinary tract, alimentary tracts, the tracheobronchial tree, a cerebral aqueduct or the genitourinary system. Stents may be used in both humans and animals.
There are typically two types of stents: self expanding stents and balloon expandable stents. Self expanding stents automatically expand once they are released and assume a deployed, expanded state. A balloon expandable stent is expanded using an inflatable balloon catheter. The balloon is inflated to plastically deform the stent. Balloon expandable stents may be implanted by mounting the stent in an unexpanded or crimped state on a balloon segment of a catheter. The catheter, after having the crimped stent placed thereon, is inserted through a puncture in a vessel wall and moved through the vessel until it is positioned in the portion of the vessel that is in need of repair. The stent is then expanded by inflating the balloon catheter against the inside wall of the vessel. Specifically, the stent is plastically deformed by inflating the balloon so that the diameter of the stent is increased and remains at an increased state. In some situations, the vessel in which the stent is implanted may be dilated by the stent itself when the stent is expanded.
The Palmaz-Schatz(trademark) stent, which is disclosed in the Handbook of Coronary Stents by Patrick W. Serruys et al. (Martin Dunitz, LTD 1998), is an example of a balloon expandable stent that had been implanted in hundreds of thousands of patients. The Palmaz-Schatz(trademark) stent, like other known stents, has certain limitations. These include, but are not limited to: (i) low stent-to-vessel ratio uniformity, (ii) comparative rigidity of the stent in a crimped as well as deployed state, and (iii) limited flexibility making delivery and placement in narrow vessels difficult. Stent-to-vessel ratio generally refers to the degree that the vessel wall is supported by the stent in its expanded state and preferably should be uniform throughout the length of the stent. Furthermore because the Palmaz-Schatz(trademark) stent consists of one or more bridges that connect a number of consecutively slotted tubes, there are a number of bare areas in the vessel after the expansion of the stent. These shortfalls are common to many stents. Id. at 36.
The present invention is directed to expandable stents that have geometries that allow them to be readily crimped onto a balloon delivery device. In one embodiment, the stent may be comprised of a plurality of first helical segments having a first helical angle with respect to the longitudinal axis of the stent and a plurality of second helical segments that have a second helical angle. The helical segments are capable of expanding and contracting circumferentially, i.e., they expand or contract along the circumference of the stent. In this embodiment, when the stent is crimped, at least one portion of one first helical segment, along with at least one portion of a second first helical element, nestle between the same two portions of two separate second helical segments.
In one embodiment of the present invention, the stent is comprised of a plurality of first expandable elements and a plurality of second expandable elements. The first expandable element may have a segment that nests within another segment of the same first expandable element. In some embodiments, the first expandable elements are joined together by struts to form first helical segments and the second expandable elements are joined together by struts to form second helical segments. The first and second helical segments may have different helical angles or different pitches. In some embodiments, the first and second helical segments share common struts.
In some embodiments of the present invention, the stent may be comprised of a plurality of cells. Each cell may be comprised of first and second elements that are alternatively joined together (i.e., each first element is joined to two second elements and each second element is joined to two first elements to form a polygon). The polygon may be amorphous or may have a definite shape. When the stent is crimped a portion of each first of the elements that make up the cell nestles between portions of the second elements of the cell. In some embodiments, the first and second elements may touch each other when the stent is crimped. A plurality of struts joins the cells to form a stent body. In addition portions of a first element may nest within other portions of the same first element and a portion of a second element may also nest within a portion of the same first element.